This invention relates in general to textile processing or manufacturing equipment. More specifically, it relates to an optical sensing system to determine the angular orientation of weft threads in a moving web of a textile.
As is well known, it is important in the manufacture of many textiles to ensure that the weft threads are substantially straight and perpendicular to the longitudinal or warp threads. In modern, high speed textile manufacturing machines, it is essential to have a system that automatically senses the orientation of weft threads and in response generates a control signal for mechanisms which can correct the two major types of irregularities, skewed and bowed threads. To correct skewing, the axis of rotation of a pair of straight rolls is shifted to adjust the relative travel speeds of the web selvages. To correct bowing, a pair of bowed rolls are rotated to advance or retard the speed of the central portion of the web with respect to both selvages. In practice, the quality and cost of the textile is directly dependent on the sensitivity and reliability of the sensing system.
A wide variety of optical sensing systems are known. In general they direct a beam or beams of light through the textile and utilize photodetectors to receive and convert the light into an electrical control signal for the straightening rollers. For example U.S. Pat. Nos. 2,106,611; 2,106,612; 2,196,893; 2,209,220; 2,492,737, and 2,623,262, all assigned to the General Electric Company, describe optical weft detection systems utilizing rotating chopping disks, inclined slots, rotating rectangular beams of light and other arrangements to sense the orientation of weft threads and convert this information into an electrical signal. The signal may be a voltage, as in the '737 and '262 patents, or a frequency, as in the '611, '612, and '893 patents. The '612 patent, for example, interrogates the web with two beams of light chopped by a slotted rotating drum that surrounds the light source. A cylindrical lens focuses the chopped beams onto two fixed, mutually inclined slots. A photodetector on the opposite side of the web from both the slots and the light source measures the differences in the frequency of the transmitted light. This difference is indicative of the orientation of the weft threads.
U.S. Pat. No. 3,077,656 to Mahlo describes a more recent optical sensor. It employs two mutually inclined slots formed in a sensing head located on the opposite side of the web from a light source. The slot widths are approximately equal to the width of a thread. The light transmitted through a slot generates a maximum signal on a photosensitive plate located behind the slot when the weft threads are aligned with the slot. The frequency of the resulting electrical signal is not important. Measurement of the signals at both slots indicates the weft thread orientation. One major difficulty with this system is that it is not highly accurate. A particular problem is that the system may have a "dead spot"; that is, it will not accurately define the angle of weft threads having a range of angular orientations intermediate those of the slots. To reduce this problem, a more recent model employs four slots with a closer angular separation. As before, each slot feeds light to a single cylindrical lens which focuses the light on a photosensitive plate.
Even with the additional slots, the Mahlo system is not effective for what can be termed "difficult" fabrics; that is, ones where the fabric is sufficiently dense that light will not penetrate it, where the thread pattern is highly irregular, or where the thread size or thread count per inch is at an extreme value. To attempt to deal with these fabrics, yet another Mahlo system utilizes a rotating scanning head with a single slot. Correlation between the angular position of the head and the signal generated by the light passing from the fabric through the slot gives an indication of the weft thread orientation. While this system may provide more precise information than fixed head systems, it has maintenance and cost disadvantages associated with rotating elements and the necessity of measuring and correlating the position of the rotating member with the output signal.
Another optical system described in Japanese Publication Document No. 88382 utilizes a screen that carries a set of gratings or line patterns. The gratings in a column of the screen correspond to a given weft thread count per inch. Within each column, each grating has a different "pitch" or angle of inclination. The illuminated thread pattern projected on a column of the gratings creates a series of moire patterns. One pattern is characteristic of the weft threads being aligned with the grating. This system has the disadvantage that the operator must determine the thread count and make corresponding adjustments to select the proper column. This system is therefore not fully automatic. It is also susceptible to error in making the initial thread count. If the thread count is incorrect to any significant degree, the moire pattern is likely to disappear.
It should be noted that all optical systems must operate in a high "noise" environment, that is, signals due to naturally occurring irregularities in the size and direction of the weft threads, variations in the thread count, variations in the fabric travel speed, and large magnitude variations due to large irregularities in the fabric. Optical systems also frequently generate errors due to aging of components of the system, particularly the light source.
In contrast to the foregoing optical systems, the assignee of the present invention has developed a mechanical system for detecting the orientation of weft threads. It utilizes a pivoted sensing head with wheels that engage a moving textile web. The assembly pivots in response to variations in the cloth that correlate with the lay of the weft threads. This mechanical system is highly sensitive and has proven to be reliable for a wide range of fabrics, including dense fabrics such as corduroy, denim, and terry which cannot be handled by an optical system. The mechanical sensor, however, may not perform as well when the textile has variations in its cross machine elasticity or on complex fabrics such as ones having a series of longitudinal cables or braids linked laterally by loose patterns of threads.
It is therefore a principal object of this invention to provide an optical sensing system that accurately and reliably determines the angular orientation of weft threads including weft threads in complex or difficult fabrics.
Another object of the invention is to provide an optical sensing system with the foregoing advantages that also has a good angular sensitivity with no dead spots.
Another significant object of the invention is to achieve accurate and reliable weft angle detection with no mechanical scanning arrangement and its attendant cost and maintenance problems.
Still another object of the invention is to provide an optical sensing system that does not rely on optical interference effects and is not sensitive to variations in the thread count.
A further object of the invention is to provide an optical sensing system that accepts and automatically inspects fabrics with a wide range of thread counts moving over a wide range of web speeds.
A still further object is to provide a sensing system that is substantially insensitive to large but non-recurring variations in the fabric.
Yet another object of the invention is to provide a sensing system that automatically accommodates for variations in the characteristics of the fabric and variations in the sensing system itself.
Another object is to provide an optical sensing system utilizing simple, low cost optical components.